In U.S. Pat. No. 3,792,086, a one-step process for the oxidation of propylene into acrylic acid at temperatures up to 300.degree. C. is disclosed. The catalyst used in this process is a supported palladium metal, alone or admixed, alloyed or in solid solution with a minor amount of a further metal, e.g., a Group IB metal such as silver or gold, and phosphoric acid. The use of H.sub.3 PO.sub.4 is essential in this process in order to obtain the desired results. Unfortunately, the use of the phosphoric acid has inherent disadvantages among which equipment corrosion encountered in feeding the acid to the reaction zone and deterioration of the silica and alumina catalyst supports by the acid can be mentioned.
I have now found that the use of H.sub.3 PO.sub.4 can be avoided by converting propylene into acrylics by a multi-step route. In the first step, propylene is oxidized in the presence of a palladium metal catalyst and acetic acid to produce allyl acetate and water. In the second step, the allyl acetate is hydrolyzed in the presence of an acid catalyst to produce allyl alcohol and acetic acid.
The acetic acid produced in the second step is recycled for use in the propylene oxidation stage. The next step in my process involves the oxidation of the allyl alcohol in the presence of a supported palladium-copper of palladium-silver metal catalyst in the vapor phase and at low temperatures to produce acrolein and/or acrylic acid in good yield.
In McClain et al, U.S. Pat. No. 3,739,020, a vapor phase process for the preparation of carboxylic acids by the direct oxidation of 2-4 carbon atom alkanols in the presence of a solid palladium metal containing catalyst at temperatures up to 150.degree. C. is disclosed. The palladium catalyst used in this process can be in admixture, alloyed or in solid solution with other metals such as, e.g., gold. The patent also teaches that the catalyst can additionally carry as a promoter, a compound of various metals including copper, most often in the oxide, carbonate or acetate form.
It is known to oxidize primary alcohols with air, oxygen or oxygen producing substances to produce the corresponding aldehydes or carboxylic acids. However, if unsaturated primary alcohols are involved, secondary reactions usually take place to a relatively great extent because the oxidizing agent also attacks the double bond and results in extensive decomposition of the molecule, producing, e.g., formaldehyde, glyoxal, formic acid or oxalic acid. Low yields are generally obtained because the carboxylic acids are aldehydes are very reactive compounds, which themselves are subject to addition reactions and particularly to polymerization. In U.S. Pat. No. 3,449,413, it is taught that these problems can be substantially overcome by carrying out the oxidation of the alcohols in an alkaline aqueous medium containing a mixture of cuprous oxide or cuprous hydroxide and a noble metal and/or its oxide or hydroxide.
The prior art also teaches that allyl alcohol can be oxidized to acrolein and/or acrylic acid at high temperatures using a copper or silver catalyst or palladium salts promoted with a combination of other metals. Thus, U.S. Pat. No. 2,042,220 teaches using a silver catalyst at temperatures from 360.degree.-550.degree. C.; CA 50, 11940 (1955) describes a silver catalyst at temperatures of 200.degree.-400.degree. C. with the best yield of acrolein being 53% at 340.degree.-350.degree. C.; CA 40, 4348 describes the use of a silver catalyst at 200.degree.-240.degree. C. with a conversion of 76% of theory; CA 63, 8184 teaches the production of acrolein using a silver or copper catalyst at 200.degree.-530.degree. C., the copper causing more of the allyl alcohol to burn into C0.sub.2 ; and CA 70, 87049 teaches the use of a salt or coordination compound of palladium such as a mixture of palladium acetate, lithium acetate and cuprous acetate.
U.S. Pat. No. 3,862,236 teaches that allyl alcohol can be isomerized into propionaldehyde, a saturated aldehyde, by passing the allyl alcohol in the gas phase over a supported palladium and gold catalyst at a temperature of 50.degree.-250.degree. C.
It is the object of this invention to provide a new process for the oxidation of allyl alcohol into acrylics and to provide a multi-step continuous process for converting propylene into acrylics which avoids the use of phosphoric acid, has lower heat requirements than prior art processes and provides high overall conversion and selectivity. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.